One of the major problems in the field of cryptography is to restrict access to transmitted information such that only its intended recipient can correctly understand it. Modern day encryption techniques rely on a set of specific parameters, called a key, to be provided together with the actual message as an input to an encrypting algorithm. Similarly, for decryption, the key needs to be input together with the encrypted message to the decrypting algorithm to arrive at the original message. The encrypting and decrypting algorithms are often publicly known or announced and so the security of the encrypted message depends entirely on the secrecy of the key.
The key typically comprises a randomly chosen, sufficiently long string of bits. Once it has been determined, subsequent communication involves sending encrypted messages over any channel (even a public channel) whose continual security against eavesdroppers is not important. However, in order for the sender and recipient who share no secret information initially, to share a secret key it is necessary in classical key based communication protocols to transmit some key determining information along a secure and reliable channel. The security of any such classical key-based communication protocol is dependent on how difficult it is for an eavesdropper to derive the key from the transmitted key determining information. Furthermore, the sender and recipient have no way of ensuring that they can be certain of telling that any eavesdropping has taken place. Accordingly, no matter how difficult deriving the key may be, in principle this is an inherent weakness of all such classical key-based communication protocols.
Another inherent weakness is that in general, if the key length is shorter than the message length, it is not possible to give an absolute guarantee that useful information about the original text or key or both cannot be obtained by an eavesdropper who cryptanalyses the encrypted text.
One attempt to address the first of these problems uses the mathematical technique of public and private key protocols. In these protocols, messages are sent without the senders and recipients having agreed on a secret key prior to sending the message. Rather, this protocol works on the principle of encryption/decryption with two keys, one public key to encrypt it, and another private one to decrypt it. Everyone has a key to encrypt the message but only one person has a key that will decrypt it again so anyone can encrypt a message but only one person can decrypt it. The systems avoid the key distribution problem described above as public keys are widely distributed with no security. However, the security of these asymmetric security protocols unfortunately depends on unproven mathematical assumptions, such as the difficulty of factoring large integers (RSA—the most popular public/private key protocol—gets its security from the difficulty of factoring large numbers). There is a danger that mathematicians/computer scientists will probably come up with significantly faster procedures for factoring large integers and then the whole privacy and discretion of public/private key protocols could disappear instantly. Indeed, recent work in quantum computation shows that quantum computers will be able to factorize much faster than classical computers, so that, for example, RSA would be highly insecure if and when large quantum computers are built.
A new type of cryptography called Quantum Key Distribution (QKD) has emerged more recently. Existing quantum key distribution protocols fall into two basic classes: those requiring entanglement and those that do not. Entanglement-based protocols which use quantum computers to manipulate the qubits sent and received have some security and efficiency advantages in theory, and also can be used to implement quantum key distribution over long distances efficiently. However, no quantum computer has yet been built. Also, sources of entangled photons with flux rates comparable to those of sources of unentangled photons, currently do not exist. Hence, secure entanglement-based quantum key distribution (QKD) is presently difficult to implement in practice. It may well be that entanglement-based QKD will always be less efficient than non-entanglement-based QKD for a range of important applications.
There are several quantum key distribution protocols which do not require entanglement. The most commonly implemented is the Bennett-Brassard 1984 protocol, usually referred to as BB84 (Bennett C. H. and Brassard, G. Quantum Cryptography: Public Key Distribution and Coin Tossing. Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing. IEEE, New York. pp. 175-179 [1984]). Others include the B92 protocol, the 6-state protocol considered by Bruss, and the Goldenberg-Vaidman protocol.
One of the key attributes which underpins all QKD protocols is that, according to quantum physics, observation generally modifies (disturbs) the state of what is being observed. By following protocols that exploit this property, two parties can set up a communication scheme that allows them to detect any eavesdropping by a third party, because they will be able to observe the disturbances introduced by any such third party.
The BB84 QKD protocol relies on a quantum communications channel between the sender and recipient being established such that quantum information such as light photons polarised into one of several states can be sent over the channel (e.g. a telecom optical fibre or beam transmitted through the atmosphere or through space). Also the protocol makes use of a public channel such as a radio channel over which public messages can be sent. The object of the protocol is to generate a random shared key kept secret from eavesdroppers, even when there is some level of eavesdropping on the communications that comprise the protocol. If so, the key can be rejected and the procedure repeated until a non-disturbed key has been received. No secret information is disclosed even if the eavesdropper happens to listen in because the actual secret information message is never sent until the key has been received without any eavesdropping. Once a key has been established between the sender and recipient, the secret information message can be coded at the sender with the shared secret key, transmitted over the public channel and securely decoded by use of the same shared secret key at the recipient.
The way in which the key is distributed from the sender to the recipient securely is described mathematically in various different texts, see for example Chapter 7: ‘Quantum Cryptography’ in ‘Quantum Computation and Quantum Information’ by Michael A. Nielsen and Isaac L. Chuang, 2000, publisher: Cambridge University Press; ISBN: 0521635039. However, the following example set out in the introduction to Quantum Cryptography at the Qubit.org website helps to understand the basic principle of how the protocol works.
“The system includes a transmitter and a receiver. A sender may use the transmitter to send photons in one of four polarisations: 0, 45, 90, or 135 degrees. A recipient at the other end uses the receiver to measure the polarisation. According to the laws of quantum mechanics, the receiver can distinguish between rectilinear polarisations (0 and 90), or it can quickly be reconfigured to discriminate between diagonal polarisations (45 and 135); it can never, however, distinguish both types. The key distribution requires several steps. The sender sends photons with one of the four polarisations which are chosen at random. For each incoming photon, the receiver chooses at random the type of measurement: either the rectilinear type or the diagonal type. The receiver records the results of the measurements bat keeps them secret. Subsequently the receiver publicly announces the type of measurement (but not the results) and the sender tells the receiver which measurements were of the correct type. The two parties (the sender and the receiver) keep all cases in which the receiver measurements were of the correct type. These cases are then translated into bits (1's and 0's) and thereby become the key. An eavesdropper is bound to introduce errors to this transmission because he/she does not know in advance the type of polarisation of each photon and quantum mechanics does not allow him/her to acquire sharp values of two non-commuting observables (here rectilinear and diagonal polarisations). The two legitimate users of the quantum channel test for eavesdropping by revealing a random subset of the key bits and checking (in public) the error rate. Although they cannot prevent eavesdropping, they will never be fooled by an eavesdropper because any, however subtle and sophisticated, effort to tap the channel will be detected. Whenever they are not happy with the security of the channel they can try to set up the key distribution again.”
This example assumes communications will be perfect and noise-free in the absence of eavesdropping. However, in practice, it is known that no communications system is perfect and there are likely to be distortions provided in the transmitted qubits caused by the quantum channel or even by eavesdroppers. These errors are corrected by use of error correction and privacy amplification techniques which are known to the skilled addressee, see for example Chapter 7: ‘Quantum Cryptography’ in ‘Quantum Computation and Quantum Information’ by Michael A. Nielsen and Isaac L. Chuang, 2000, publisher: Cambridge University Press; “Generalised Privacy Amplification”, C. Bennett et al, IEEE Trans. Info. Theory Vol 41 (1995) pp 1915-1923; and “Secret Key Agreement by Public Discussion from Common Information”, U. Maurer, IEEE Trans. Info. Theory, Vol 39 (1993) pp 733-742.
The present invention aims to overcome or at least substantially reduce the problems described above and in the references mentioned above and to provide a robust method of distributing a secret key by transmitting quantum information.
The present invention has arisen from an appreciation that the prior art methods have required there to be a limited number (in the simplest cases, two or three) of possible bases or frames of reference for determining the state (of a pair of orthogonal states) to which a quantum element belongs. This limitation has been necessary with the prior art because of the underlying requirement to know which bases are correct, as has been discussed above and illustrated by the described prior art example. However, the present inventors have appreciated that it is not necessary to restrict the method of distributing a shared key to a small predetermined number of bases but rather an almost infinite number of different bases can be used. This is made possible by transmitting some information about a subset of the transmitted qubits, for example some information about the bases used for that subset of the transmitted qubits, which enables determination of the level of eavesdropping, if any. Then by using transmitted basis information about the rest of the transmitted qubits and by carrying out statistical analysis on that information, it is possible for the sender and recipient to derive correlated bit strings about which eavesdroppers can have little or no information. Then the discrepancies between the versions of the correlated bit strings at the sender and recipient caused by use of such large numbers of bases can then be reconciled using existing privacy amplification and error correction techniques to derive a shared secret key.
More specifically according to one aspect of the present invention there is provided a method of establishing a shared secret random cryptographic key between a sender and a recipient using a quantum communications channel, the method comprising: generating a plurality of random quantum states of a quantum entity, each random state being defined by a randomly selected one of a first plurality of bases ill Hilbert space; transmitting the plurality of random quantum states of the quantum entity via the quantum channel to the recipient; measuring the quantum state of each of the received quantum states of the quantum entity with respect to a randomly selected one of a second plurality of bases in Hilbert space; transmitting to the recipient composition information describing a subset of the plurality of random quantum states; analysing the received composition information and the measured quantum states corresponding to the subset to derive a first statistical distribution describing the subset of transmitted quantum states and a second statistical distribution describing the corresponding measured quantum states; establishing the level of confidence in the validity of the plurality of transmitted random quantum states by verifying that the first and second statistical distributions are sufficiently similar; deriving a first binary string and a second binary string, correlated to the first binary string, respectively from the transmitted and received plurality of quantum states not in the subset; and carrying out a reconciliation of the second binary string to the first binary string by using error correction techniques to establish the shared secret random cryptographic key from the first and second binary strings.
Reconciliation has primarily been used for overcoming errors in transmitted data between the sender and the intended recipient, however the present invention extends its use to act as a foundation for deriving two shared secret keys from two correlated data strings present at the recipient and the sender after quantum transmission of the data making up the strings. The appreciation that reconciliation/amplification techniques can be used in quantum cryptography in this way means that the number of bases used for encoding the state of the Qubits is not limited to a small finite number (two or three) as the prior art but becomes practically infinite.
The present invention describes an idea for refining earlier quantum key distribution schemes, based on the observation that it is not necessary for the sender and recipient to identify a string of qubits on which the recipient carried out measurements in a basis containing the qubit prepared by the sender. A problem with existing practical implementations of quantum key distribution is that, given the actually attainable bounds on the level of eavesdropping (which are nonzero, even if there is no actual eavesdropping, because of the presence of noise on the quantum channel) they generate a shared secret key at a relatively low bit rate. The present invention potentially offers a higher bit rate for secret key generation. Also, existing protocols have some potential security weaknesses arising from the fact that the sender's prepared qubits and the recipient's measurement choices are drawn from short lists of possibilities, which are known to (and if not, are deducible after some eavesdropping by) the eavesdropper. By allowing a much larger set of choices, the present invention reduces these potential weaknesses. Further, the existing protocols that are presently practical do not have the property of deniability. That is, the sender and recipient may, if interrogated after the fact, be able to generate a fake transcript of the protocol which produces a fake secret key of their choice, with less risk that their misrepresentation can be exposed even if their interrogator has eavesdropped on them during the key distribution protocol. The present invention describes protocols which are also presently practical but which potentially allow a greater degree of deniability.
The term ‘quantum entity’ is intended to mean any entity which is capable of having measurable quantum characteristics. For example, the embodiment of the present invention is described with reference to photons being the quantum entity with their measurable quantum characteristics being polarisation of the photon. However, other types of entities are also covered by the term, for example electrons and atomic nuclei where, in both cases, the spin degree of freedom can provide the measurable quantum characteristic.
Whilst existing prior art schemes such as BB84 QKD protocol are secure in principle, and can be made secure in practice, they potentially require rather more resources (i.e. more quantum and/or more classical communication per secure key bit generated) than the protocol of the present invention.
Preferably the first and second plurality of randomly selected bases in Hilbert space each comprise at least four random bases The higher the number of sets of bases the greater the potential level of security and hence potential benefit provided by the present invention.
The selecting step may comprise generating and measuring a first plurality of bases in two-dimensional Hilbert space. However, the selecting step may alternatively comprise generating and measuring a first plurality of bases in a real sub space of two-dimensional Hilbert space. This alternative implementation has some potential advantages in that it offers different and potentially advantageous tradeoffs between efficiency and security. It is also easier to implement in practice for some physical realisations.
More specifically, one advantageous way of implementing the establishing step would be to determine the degree of difference between the first and second statistical distributions; and to accept the security of the channel if the degree of correlation between the two distributions is greater than a threshold level. The use of statistical distributions provides a fast mathematical way of automatically assessing the degree of deviation of the measured results from the transmitted results. Furthermore, using statistical distributions allows a degree of error to be accommodated without the need for error correction techniques prior to the comparison step.
Preferably the method further comprises selecting the value of the threshold level. This advantageously enables the method to permit a level of eavesdropping which potentially may exceed that tolerable with prior art protocols As a result, the user can grade the level of information to be communicated and can determine a corresponding threshold level. Clearly, the lower the threshold, the more chance there is of a key being established on the first attempt. Also as transmission errors would also contribute to imperfect comparison results, the threshold can be set to accommodate such errors.
It is to be appreciated that, in the present embodiment, the step of generating a plurality of random quantum states comprises generating quantum states that are part of a two-dimensional system. However, the present invention can also extend to higher-dimensional systems where the generation step may comprise generating random quantum states describing more degrees of freedom. For example, say in three dimensions, different characteristics to be considered of say an atomic quantum entity could be the spin of a nucleus being greater than ½, the position wave function of one of its quantum objects (such as a photon, electron, nucleus, etc.) that is constrained so that its position lies in some fixed finite dimensional space, or the state of an excited atom which is constrained to lie in the space defined by some fixed finite set of energy levels.
For most practical applications of the present invention, it is preferable for the pluralities of bases to be approximately uniformly separated. If there are a large number of bases in the first plurality of bases in Hilbert space, then this can be achieved by choosing the bases randomly. However, in cases where there are fewer bases to choose from, the uniform separation can be ensured by choosing bases in a specific geometric configuration (for example one defined by a platonic solid) in which they are roughly uniformly separated. This applies to the complex version of Hilbert space version of the method. In the real subspace version for any number of N bases, they can be chosen to be precisely uniformly separated by taking the vectors in the Great Circle to be separated by angle pi/N.
The method may further comprise temporarily storing the received quantum states of the quantum entity prior to carrying out the measuring step. This enables the sender to transmit some specific information about the sender's bases which can then be used by the recipient in the measurement of the stored qubits. Also, in the absence of eavesdropping, storage advantageously allows the sender and recipient to generate a random shared key at the rate of one bit per photon qubit transmitted. Another major advantage of storage is that it offers a greater level of security to the communications protocol.
The second plurality of bases may be determined independently of the first plurality of bases. Whilst this clearly makes the method more complicated in that the reconciliation step has more work to do, it may however advantageously improve some aspects of the security of the method.
Preferably the established shared secret key is of the same size as the magnitude of the message which is to be encrypted. This is because as with all uses of a one-time pad encryption scheme (a.k.a. the Vernam cipher) it provides the maximum possible information-theoretic security. Of course it is also possible to accept imperfect security if the tradeoff is that this allows one to send a longer message. In addition, it is also possible to use a shared secret key K1 generated by the quantum scheme to encrypt another key K2 of the same length which is used in some standard classical cryptographic scheme, and then use this classical scheme to send messages of longer length. The security here is imperfect but could be very good: it relies on the facts that K2 is completely concealed from eavesdroppers, and that (if K2 is long) the classical scheme may be very hard indeed to break without knowledge of the key used (K2).
The present invention is practical and can be implemented with existing technology. In particular, there is no requirement for even small-scale quantum computers for implementation. Rather, the present invention can be implemented with single photon sources or weak photon pulses, and does not require an entangled photon source. This means that a relatively high qubit transmission rate is practical. A presently preferred embodiment of the present invention has the following potential advantages over existing similarly practical quantum key distribution schemes such as the B384, B92 and 6-state quantum key distribution protocols. First, it is potentially more efficient, in the sense that it allows more bits of the secret key to be generated per qubit sent, for a given level of eavesdropping. Second, it has a potentially higher security threshold, in the sense that it allows a secret key to be generated in the presence of a higher level of eavesdropping or noise than existing protocols. Third, it may be more secure, in the sense that it is resistant (or more resistant) to a wider variety of active eavesdropping attacks (in which physical states other than those used in the protocol are introduced into the quantum channel by the eavesdropper) and to other forms of sabotage. Fourth, it may allow the sender and recipient a higher level of deniability than those existing protocols which do not require quantum computation.
According to another aspect of the present invention there is provided a secure communications method for conveying a message from a sender to an intended recipient, the method comprising establishing a shared secret random cryptographic key between a sender and a recipient using a quantum communications channel using the method described above; using the shared secret key as a one-tine pad for secure encryption of the elements of the message at the sender; transmitting the encrypted message to the intended recipient using a conventional communications channel; and using the shared secret key as a one-time pad for secure decryption of the encrypted elements of the message at the recipient.